1. Technical Field
The present disclosure relates to an image forming apparatus with a fixing unit.
2. Description of Related Art
Energy-saving image forming apparatuses are demanded in accordance with recent increasing momentum toward conservation of the global environment. On the other hand, full-color image forming apparatuses are also demanded producing both black-and-white and full-color images in accordance with recent colorization of office documents.
Electrophotographic image forming apparatuses, including at least one of a function of copier, printer, facsimile, and plotter, equipped with a fixing device are well known. Various types of fixing devices have been proposed which suppress defective fixing of a toner image on a recording medium. For example, a fixing device including a fixing roller and a pressing roller is well known. The fixing roller is comprised of a heat roller to be heated by a heat source. The pressing roller and the fixing roller press against each other to form a fixing nip therebetween in which a toner image is fixed on a recording medium. Such a fixing process is a so-called heat roller process.
A fixing device employing the heat roller process generally includes a fixing roller and a pressing roller. The fixing roller is adapted to melt a toner including a thermoplastic resin (hereinafter “thermoplastic resin toner”) on a recording medium. The pressing roller presses against the fixing roller so that the recording medium can be sandwiched therebetween. The fixing roller is a cylindrical member containing a heating element on the central axis thereof. The heating element may be, for example, a halogen lamp which generates heat upon application of a predetermined voltage. Because the heating element is disposed on the central axis of the fixing roller, heat generated by the heating element is uniformly radiated by the inner wall of the fixing roller. Therefore, the temperature distribution of the outer wall of the fixing roller is uniform in a circumferential direction. The outer wall of the fixing roller is heated to a proper temperature for fixing toner images, for example, 130 to 200° C. The fixing and pressing rollers rotate in the opposite direction while being heated and pressed against each other so that the recording medium having the thermoplastic resin toner thereon is sandwiched therebetween. In the fixing nip where the fixing roller and the pressing roller meet and press against each other, the thermoplastic resin toner is melted by heat from the fixing roller and fixed on the recording medium.
The heat roller process has a disadvantage that a large amount of energy is wasted. The heat roller process has another disadvantage that it takes a relatively long time to heat the fixing roller to a predetermined fixing temperature since the image forming apparatus is powered on. In a case in which the image forming apparatus has a high linear speed (hereinafter “high-speed machine”), the following problems further arise. It is difficult to secure sufficient time for fixing the thermoplastic resin toner on a recording medium in the heat roller process in which heat and pressure are simultaneously applied to the toner. Therefore, in high-speed machines, the fixing roller is required to provide a higher temperature and a higher pressure, which results in a large power consumption. Also, the fixing roller is required to have a larger diameter so as to secure sufficient time for fixing the thermoplastic resin toner on a recording medium, which results in a larger heat capacity and a larger power consumption in the fixing roller. In this case, because non-image areas are unnecessarily heated, the recording medium undesirably curls up by the unnecessary heat.
In view of these situations, an attempt to reduce the fixing temperature in the fixing nip and another attempt to fix a toner image on a recording medium without heat have been made. For example, Japanese Patent Application Publication No. 58-126561 describes a pressure fixing process in which a toner image is fixed on a recording medium by pressure without heat energy. Japanese Patent Application Publication No. 58-086557 describes a pressurization toner including 30 to 70 parts by weight of a bis(fatty acid amide) and 30 to 70 parts by weight of a polyethylene wax as binding agents.
Japanese Patent Application Publication No. 2009-251021 describes a pressure fixing process in which a toner image on a recording medium is preliminarily heated and softened before being fixed thereon by pressure, to improve fixing strength and image gloss.
Japanese Patent Application Publication No. 59-119364 describes a process in which a toner is dissolved in a solvent which can dissolve silicone oils. The toner is fixed on a recording medium without heat. Japanese Patent No. 3290513 describes a wet fixing process using a fixing liquid. The fixing liquid is an O/W emulsion in which a solvent which is insoluble or poorly soluble in water is dispersed in water. The fixing liquid can dissolve or swell toner. The fixing liquid is sprayed or dropped on the surface of a recording medium so that an unfixed toner image is dissolved or swelled thereon. The toner image is fixed on the recording medium by drying the recording medium.
Image forming apparatuses using a toner including a resin in which phase transition is induced by pressure (hereinafter “pressure-induced phase transition resin toner”) have been also proposed. The pressure-induced phase transition resin is generally fluidized under pressure and is also called as baroplastics. For example, ethylene-based unsaturated compounds prepared by mini emulsion processes or living radial polymerizations and polyester block copolymers including amorphous blocks and crystalline blocks are well known as the pressure-induced phase transition resins.
Japanese Patent No. 4582227 describes an image forming method in which a pressure-induced phase transition resin toner including a polyester block copolymer is fixed at a fixing temperature of 15 to 50° C. and a fixing pressure of 0.1 to 5.0 MPa. Japanese Patent No. 4525828 describes an image forming apparatus in which a pressure-induced phase transition resin toner is fixed with a maximum fixing pressure of 5.0 MPa without heat. Japanese Patent Application Publication No. 2009-053318 describes an image forming apparatus in which a pressure-induced phase transition resin toner remaining on an image bearing member (photoreceptor) is removed by a cleaning blade. Japanese Patent Application Publication No. 2010-191197 describes a full-color image forming apparatus including multiple imaging units containing different-color pressure-induced phase transition resin toners. In this full-color image forming apparatus, a full-color toner image formed on an intermediate transfer belt is simultaneously transferred onto and fixed on a recording medium. Generally, pressure-induced phase transition resin toners have an advantage over thermoplastic resin toners in terms of energy saving.
However, a full-color image forming apparatus including multiple imaging units containing different-color pressure-induced phase transition resin toners has the following problem.
In such a full-color image forming apparatus including multiple imaging units containing different-color pressure-induced phase transition resin toners, multiple toner images are transferred onto an intermediate transfer medium or a recording medium directly. Thereafter, the toner images are fixed on the recording medium upon application of a high pressure thereto. The high pressure can be applied to the toner images with a pair of metallic rollers. The toner images are applied with a uniform pressure to be uniformly fixed on the recording medium.
In a full-color toner image in which multiple toner images are superimposed on one another, the toner pile height varies with location. Due to the non-uniform toner pile height, the full-color toner image is applied with a non-uniform pressure in the fixing nip and fixed on a recording medium with non-uniform fixing strength. FIG. 1 is a schematic view illustrating a related-art process of fixing a full-color toner image on a recording medium. A full-color toner image includes an image area having a high toner pile height formed with multiple color toners and an adjacent image area having a low toner pile height formed with a single color toner. In image forming apparatuses using thermoplastic resin toners, the fixing member, such as a fixing roller or a fixing belt, generally has an elastic layer. The elastic layer can intimately contact the surface of the toner image following its non-uniform toner pile height. Thus, the toner image can be uniformly applied with heat and pressure.
By contrast, in image forming apparatuses using pressure-induced phase transition resin toners, the fixing members, i.e., the metallic rollers, generally do not have an elastic layer. Therefore, the fixing members cannot intimately contact the surface of the toner image following its non-uniform toner pile height. As a result, the image area having a low toner pile height is applied with only a small pressure and the pressure-induced phase transition resin toner cannot be sufficiently fluidized, as shown in FIG. 1. The pressure-induced phase transition resin toner cannot sufficiently anchor in the recording medium. Even in a case in which the pressure-induced phase transition resin toners are first formed into a film on an intermediate transfer medium by pressure before being fixed on a recording medium by pressure, the resulting film is non-uniform for the same reason described above. Some toner particles may remain in their original form without being formed into the film. In this case, remaining toner particles may contaminate the user's hands or clothes or degrade color reproducibility. To solve this problem, a higher pressure is required in the process of forming the film or fixing the film on the recording medium, which results in undesirable increase in the size and weight of the image forming apparatus.
When toner particles are applied with a non-uniform pressure, the toner particles cannot sufficiently fluidize to aggregate. Thus, the resulting image has either toner grain aggregate or voids. When the resulting image is a single-color image, the image density is low. When the resulting image is a full-color image, the fixing strength between the toner particles and the recording medium is weak and color reproducibility is poor. Because the resulting image has either toner grain aggregate or void, the surface thereof is not smooth and the image exhibits poor gloss.
When removing toner particles remaining on an image bearing member, such as a photoreceptor or an intermediate transfer medium, a cleaning blade, comprised of a urethane rubber, etc., is brought into contact with the image bearing member while applying a predetermined stress to a nip formed between the image bearing member and the cleaning blade. In a case in which pressure-induced phase transition resin toner particles are removed with the cleaning blade, the pressure-induced phase transition resin toner particles may fluidize upon application of the stress and contaminate the image bearing member. Because the fluidized pressure-induced phase transition resin toner particles may alter their shapes or form large aggregates, it may be difficult to feed them to a waste toner tank. Thus, the pressure-induced phase transition resin toner is preferably removed without application of mechanical stress.
In a tandem full-color image forming apparatus, in which multiple imaging units are arranged in tandem, toner particles transferred onto an intermediate transfer medium or recording medium may be retransferred on image bearing members (i.e., photoreceptors) on downstream sides. Imaging units on upstream sides consume much more toner particles so that the resulting image is formed with a desired amount of toner particles. Thus, not all the toner particles can be transferred onto a recording medium and remaining on the photoreceptors. Each photoreceptor cleaner in each imaging unit in the tandem full-color image forming apparatus receives much more toner particles than that in a single imaging unit in a black-and-white image forming apparatus. The photoreceptor cleaner in the most downstream imaging unit in the tandem full-color image forming apparatus receives a quite large amount of toner particles which cannot be sufficiently removed with a cleaning brush.
The above-described problems can be solved if an image area having a high toner pile height is pre-fixed on a recording medium, as illustrated in FIG. 2, so that the toner image has a uniform surface and is prevented from being applied with a non-uniform pressure in the main fixing process. For example, in a related art image forming apparatus illustrated in FIG. 3, a pressure-induced phase transition resin toner image of each color is directly transferred onto a recording medium P and immediately thereafter is applied with a pressure in each pressure fixing nip in each pressure fixing device 40. Thus, the toner pile height can be effectively reduced. However, the image forming apparatus illustrated in FIG. 3 is disadvantageous in size, weight, and manufacturing cost.